Label-free optical biological/chemical sensors are essential in the application of medical diagnosis, healthcare and environmental monitoring etc. Among all the approaches, optical microresonator-based biosensors are regarded to be very promising due to their high sensitivity to refractive index change (˜10−4-10−7 refractive index unit (RIU)), which are comparable to the sensitivity of conventional surface plasmon resonance (SPR) technique. In addition, optical microresonator-based biosensors typically have compact footprint (˜10's μm-˜100's μm), and offer potential large-scale integration with microfluidics.
Single microring resonator-based biosensors in silicon-on-insulator (SOI) have been demonstrated using either conventional microrings. The demonstrated detection limit ranges from 10−5-10−7 RIU.
However, for nearly all the demonstrated microresonator sensors, the wavelength-scanning method using wavelength-tunable lasers was considered to be the “default” technique for measuring sharp resonant wavelength shift. The wavelength-scanning method requires high-resolution wavelength tunable lasers in order to measure the sharp resonant wavelength shift, in which the detection limit is limited by the laser resolution. Furthermore, high-resolution wavelength-scanning lasers are very expensive and not suitable for point-of-care applications.